How can i develop/rekindle my work ethic? (10 Points!)?

After school these days i can never seem to get much homework or studying done. I just can't seem to motivate myself to work hard after school even though i still get pretty good grades i could be doing better. Its just that i usually dont get around to starting my work until late at night, and by then i am lethargic and too tired to accomplish much. Please Help, any suggestions would be useful!